Melting
Melting is the phase change from solid to liquid that occurs when a solid absorbs enough heat for its particles to break free of their fixed positions.
Definition
Melting, or fusion, is the phase change from solid to liquid that happens when a substance absorbs enough thermal energy for its particles to break free of their fixed positions and start to flow, occurring at the melting point, a characteristic physical property, during which the temperature holds steady while the added energy loosens the particles rather than heating them further. It occurs at the temperature where the Gibbs free energies of solid and liquid are equal, $T_m = \Delta H_{fus}/\Delta S_{fus}$, reflecting both the lattice energy that must be overcome and the entropy gained on melting, and it shifts with pressure according to the sign of the volume change in the Clausius-Clapeyron relation, while impurities lower and broaden it, a phenomenon called melting-point depression.
Example
An ice cube left on the counter melts into a puddle and a chocolate bar melts in a warm pocket, yet melting never makes something watery in general, since melted iron is still liquid iron. Ice melts at $0^\circ\text{C}$, butter at about $32^\circ\text{C}$, and iron at $1538^\circ\text{C}$, and melting $10\text{ g}$ of ice at $0^\circ\text{C}$ takes $3340\text{ J}$ of energy while the resulting water stays at $0^\circ\text{C}$ until it is all melted. Sodium chloride, with $\Delta H_{fus} = 28.2\text{ kJ/mol}$ and a melting point of $801^\circ\text{C}$, has $\Delta S_{fus} \approx 26\text{ J/mol}\cdot\text{K}$, far higher than water's, reflecting its much stronger ionic lattice compared with water's hydrogen bonds.
Key Insight
Salt spread on an icy sidewalk does not warm the ice at all; it lowers the melting point of water so the ice melts even below $0^\circ\text{C}$. Amorphous solids like glass have no true melting point and instead soften gradually as their viscosity falls over a range of temperatures, so a sharp, well-defined melting point is really a signature of long-range crystalline order, which is exactly why chemists use it to check a compound's purity and identity.